United States
Environmental Protection
Agency
Municipal Environmental Research ~
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-095 Dec. 1983
Project Summary
Application of Municipal
Sludges on Energy Crops:
A Feasibility Analysis
This study evaluates the feasibility of
using treated municipal sludge to grow
energy crops on marginal land. The use
of sludge on energy crops rather than on
agricultural crops avoids most of the
problems associated with the presence
of heavy metals and other pollutants in
sludge. This analysis shows that replacing
sludge incineration and/or landfilling
with land spreading on energy crops
can result in dollar and energy credits to
municipalities implementing this alter-
native. The potential economic advantage
of the energy crop approach is, however,
very sensitive to local factors such as
the value of the credits gained from
eliminating incineration or landfilling
and the annual yields of the energy
crops.
Energy crop production will probably
not result in a net revenue for a city,
since the costs of spreading the sludge
and processing the energy crop are
likely to be greater than the dollar value
of the crop. And because the present
market value of farm products is greater
than that of energy crops, sludge
disposal on the latter is slightly more
costly than on agricultural crops. But
energy crops do generate clean, renew-
able energy, whereas agricultural crops
consume energy. The analysis presented
here is valid for cities with populations
of 50,000 to several million, and it
applies essentially to all regions of the
United States.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This study examines the use of sewage
sludge for energy crop production, an
activity with the potential for solving
three societal problems at once: municipal
(sewage sludge) disposal, low soil produc-
tivity, and energy production. The objectives
of the project were as follows:
1. To assess the economic feasibility
of using municipal sludge on land
devoted to the production of energy
crops;
2. To determine the net energy balance
of using sludge to generate energy
crops; and
3. To assess the potential impact of
this sludge disposal method on
a national scale.
The term "energy crop" refers to any
form of biomass (agricultural, woody, or
grass crops) grown specifically for its
energy value. The energy may be recovered
by direct combustion or through conversion
to another form.
Application of municipal sludge to
agricultural land or recreational areas is
recognized as one of the promising
methods for ultimate sludge disposal.
Applications of these residues to food
crops, however, might create serious
problems when the crops become part of
the food chain. Disposal on recreational
areas also requires caution because of
the presence of pathogens.
The approach explored in this study is
to apply the urban sludge to land
(especially marginal land) and to generate
energy crops or crops produced exclusively
for their fuel value. Such an approach
would essentially eliminate the risk of
human or animal contact with heavy
metals and other pollutants; it would also
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generate a renewable source of energy
that would reduce the dependence of the
local company on outside energy sources.
The reclamation of marginal or submargin-
al land also offers a social benefit, but one
that is not included in the economic
feasibility study.
The study considered the situation
expected in 1985, when most municipali-
ties will have included some form of
secondary treatment and sludge manage-
ment for their wastewater treatment
plants. The economics and energy
balance of sludge disposal on energy
crops are compared with three other
methods of stabilized sludge disposal-
incineration, land-filling, and application
to agricultural crops. The comparison is
made for cities with populations of
10,000 to 4 million.
Four classes of energy crops were
considered: (1) Fast-growing, short-
rotation hardwood plantations, (2) arid
region or desert shrubs, (3) sugar crops,
and (4) grain crops. In the first two cases,
wood chips for fuel or electricity would be
the final products generated, whereas in
the latter two, ethanol is assumed to be
produced from the crops.
The economic evaluation of the energy-
crop-from-sludge concept includes the
following items: (1) the credits to the
municipality for eliminating the standard
methods of sludge disposal and for the
sales of the energy crop or its product, (2)
the costs incurred in transporting the
sludge to the plantation, (3) the costs of
storing the sludge if needed, (4) the
expense of generating and harvesting the
energy crop, (5) the price of leasing or
buying the necessary crop land, and (6)
the costs incurred in marketing the
energy crop or its product and hauling to
the point of use. The same credits and
debits apply to the disposal of sludge on
agricultural crops. No credit was taken for
improving the land as a result of sludge
application.
The energy balance evaluation takes
into account the energy credits for
eliminating incineration or landfillmg and
for generating the energy crop. Also
considered is the energy consumption
associated with transporting the sludge
and the energy crop, and with producing
and converting the energy crop. The same
energy credits and debits apply for
agricultural crops, except for the credit
associated with the crop's energy value.
The analysis is performed for five
climatic regions encompassing the
continental United States. Particular
emphasis was placed on fast-growing
hardwood crops, since in the course of
the study these crops appeared to offer
the most flexibility and the widest range
of applicability.
Methods
The method used here to evaluate the
potential of sludge disposal on energy
crops for a given municipality is outlined
in Figure 1. The municipality is character-
ized by its population and by the climatic
region in which it is located.
The following steps are involved in
evaluating the economic feasibility of the
energy crop approach. The report follows
these steps in developing the cost and/or
credit data.
1. Determine the amount of sludge
produced annually (based on popu-
lation) and the rate of sludge
production.
2. Determine the credit to the munici-
palities if incineration or landfilling
of the sludge is replaced by sludge
disposal on energy crops. Both
credits are functions of the popula-
tion and include operation and
maintenance costs for either method
of disposal. Capital recovery costs
of the inplace systems (i.e., incinera-
tion or landfilling) will still be
incurred and thus are not considered
as credits. Disposal through agricul-
tural crop production will result in
the same credits.
3. Choose energy crops suitable to the
climatic region and estimate the
crop area required to apply the
sludge on land. This and the
following steps of the method also
apply to disposal on agricultural
crops.
4. Determine the storage capacity
required on the basis of climatic
factors. Estimate the cost of storage
(a function of population and
duration of storage). This and the
following steps refer to the energy
crop plantation operations.
5. Estimate the cost of applying the
sludge to land.
6. Evaluate the production cost of the
energy crop excluding the cost of
land.
7. Estimate the cost of converting the
crop to energy.
8. Estimate the credit resulting from
the sale of the energy produced
from the crop.
9. Estimate the functional dependence
of the sludge transportation cost on
the distance from the municipality.
10. Estimate the functional dependence
of yearly land costs on the distance
from the municipality.
Population
Climatic Region
Yearly Amount
. Incineration Credit
Landfilling Credit
Cost
Cost
Crop Area
Cost of Crop
Production
Cost of Land
Cost
Credit
Figure 1. Method for evaluating the poten-
tial of sludge disposal on energy
crops.
11. Estimate the minimum distance to
be traveled from the municipality to
find land suitable for sludge disposal.
The energy crop approach will be
economically beneficial if the credits
received by the municipality are larger
than or equal to the sum of the costs
incurred in producing the energy crop.
Thus
Credit for energy crop + credit for
incineration or landfilling >costs
for energy crop production and
and conversion (1)
Identifying the terms involved in the
total costs for energy crop production in
Equation 1 becomes:
Credit for energy crop + credit for
incineration or landfillmg - (costs
for sludge storage, spreading,
crop production, and energy
conversion) > sludge transporta-
tion cost + land cost (2)
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Results of the Analysis
The study indicated that the use of
sewage sludge on marginal land to
produce energy crops is economically
feasible, conserves energy, and can
improve the environment. Energy and
monetary credits may be gained for
municipalities by replacing incineration or
landfilling with surface applications of
sludge for energy crop production. The
advantage this disposal method has for a
city is determined by local site factors.
Woody Energy Crops for Fuel
In all five regions of the country, cities
with a population of 10,000 to 4 million
could reduce sludge disposal costs by
changing from land-filling to land spreading
of sludge to produce woody plants for
energy. The cost of disposal through
energy crops does not include the cost of
processing the crop or the revenue from
energy produced.
For all regions and cities with popula-
tions larger than about 50,000, disposal
costs are less for producing woody
energy crops with sludge than for
incinerating it—assuming that sludge
pretreatment methods for land disposal
are in place and in operation. In most
cases, disposal through energy crops
reduces sludge disposal costs about
50%. But if supplementary pretreatment
must be constructed to permit land
disposal through energy crops, incinera-
tion is more economical for cities with
populations up to about 700,000. For
larger cities, replacing incineration with
disposal through an energy crop results
in an annual saving of about 30%.
Electricity from Woody Energy
Crops
Because of the high investment costs
for small power plants, electricity produc-
tion from woody crops is economically
justified only for operations of 15,000 to
20,000 acres—or specifically, for cities
with a population of 2 million or more. For
such a population center, the net credit is
smaller than for producing wood chips for
electricity than for fuel only. But larger
cities incur a larger credit when electricity
is produced. These results are influenced,
however, by the local price of electricity.
As in the case of woody fuels, the major
portion of a net credit would result from
eliminating sludge disposal by landfilling
or incineration.
Fuel from Sugar Crops
Ethanol can be produced at competitive
costs only from sugar crops in large
conversion plants. The production of fuel
from sugar crops is limited to a small
number of cities in the United States—
those that have a population of 5 million
or more and available sugar crops in
addition to those produced with sludge.
Fuel from Grain Crops
The study showed that economical
production of ethanol from grain crops
would require sludge from a population
of about 50 million. Thus to be feasible for
smaller cities, ethanol production from
sludge-produced grain would require
grain supplements from land not treated
with sludge.
Conclusions
• Growing woody biomass for fuel
on sludge-treated land is economi-
cally more advantageous than
landfilling the sludge for pities
with populations of more than
10,000. Specifically, the energy
crop approach reduces the cost of
sludge disposal by about 50% in
most cases. In semi-arid regions,
adapted species treated with sludge
would have to yield an annual 11
dry mt/ha (5 dry tons/acre).
• Growing woody biomass for fuel on
sludge-treated land is economically
more advantageous than sludge
incineration for cities with popula-
tions of more than 50,000 if land
disposal does not require new
pretreatment installations. Specific-
ally, the energy crop approach
reduces the cost of sludge disposal
by about 50% in many cases.
• Producing electricity rather than
fuel from woody energy crops is
economically attractive for cities
larger than about 2 million. The
economic advantage of sludge-
produced woody crops may reach
50% or more of the cost of sludge
disposal by incineration or landfill-
ing for cities of about 4 million or
more. The economic advantage
varies locally with the market value
of electricity in the region.
• Fuel (ethanol) production from
sugar crops is attractive only for
cities with populations larger than
about 5 million if all the raw
material is supplied from sludge-
grown energy crops.
• Fuel (ethanol) production from
grain crops is attractive only for
population centers of about 50
million if all the raw material is
supplied from sludge-grown energy
crops.
• The investment required for the
two most attractive energy crop
approaches (i.e., woody fuel and
electricity from woody fuel) strongly
favors the production of woody
fuel.
• The production of woody fuel
produces a high energy balance for
each unit of sludge applied than
does the use of woody fuel to
produce electricity. Sludge disposal
on energy crops yields a much
higher energy credit than does
disposal on land producing agricul-
tural crops.
• The economic advantage of sludge
disposal through production of
energy crops rather than incinera-
tion or landfilling is very sensitive
to credit values used in the analysis,
biomass yields, and the market
price of electricity.
The full report was submitted in
fulfillment of Contract No. 68-01-4688
by InterTechnology/Solar Corporation
under the sponsorship of the U.S.
Environmental Protection Agency.
This Project Summary was prepared by staff of InterTechnology/Solar Corpora-
tion. Warrenton, VA 21186.
Donald J. Ehreth was the EPA Project Officer (see below for present contact).
The complete report, entitled "Application of Municipal Sludges on Energy Crops:
A Feasibility Analysis." (Order No. PB 84-101 559; Cost: $16.00. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For information contact G. K. Dotson at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•aUS GOVERNMENT PRINTING OFFICE 1984-759-015/7251
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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